hi
the output power from 0-4 usually idicates that you are using Le type (Le-64.2) .
but if the out power is less than the threshold
;
1- make sure that the XFP is not faulty , through management make sure of the out optical power , if it is not between 0-4 , so maybe the XFP needs cleaning or may be you should change the patchcord you use .
2- make sure of the specifications of the XFP and that it is single mode , 1550nm , the distance not less than 60km .
3- make sure that you use single mode patch cord not multimode .
regards